Picosecond detection of electron motion

Quantum electronics consists in the generation and manipulation of quantum states encoded in the electronic degrees of freedom of a conductor. In the case of small closed systems such as quantum dots, the orbital degree of freedom can be used to encode elementary bits of quantum information. But since the typical timescales of electronic motion are way shorter than the resolution limit of standard electronic measurements, it is hard to monitor in time the electron motion inside submicrometer devices. To put some numbers, the typical internal dynamics of this kind of devices are usually higher than 1000 GHz (10¹² oscillations per second), so the resulting timescale is the picosecond, way too fast for conventional electronics, of which state-of-the-art experimental bandwidth is of the order of tens of GHz.

An international team of researchers, including an IFISC researcher (UIB-CSIC), has published a paper in the prestigious Nature Nanotechnology journal that has addressed the challenge of reaching the picosecond resolution, going beyond the limitations of previous measurement techniques. They proposed the monitoring scheme, using a quantum-mechanical resonant state formed beside a quantum dot. Resonant levels have been used for spectroscopy of quantum dots in energy domain, because the electron can escape them only if its energy matches the resonant level’s energy. Using a time-dependent voltage shifting the potential of the quantum dot, the energy resolution is converted in an interaction time, which can then be measured without bandwidth limitations. The authors experimentally demonstrate the scheme, using a dynamic quantum dot formed in a silicon nanowire. Due to dynamic tuning of the potential of the quantum dot, the electron is loaded in a superposition of the first two orbital levels of the quantum dot, and it oscillates periodically between left and right side of the quantum dot. Then the oscillatory motion is sampled in time, using a static resonant level located on a barrier of the right side and originated from a silicon-interface trap level. Measuring the current through the resonant level, generated at a well-defined time with picosecond accuracy, the oscillatory motion is detected with an unprecedented picosecond time resolution.

The ability to visualize electronic motion at the picosecond scale demonstrated in the manuscript is a new resource for the understanding of non-equilibrium electron dynamics in conductors. Further specific applications could be the encoding of information in the electron wave functions inside an electrical circuit or the possibility of high-resolution and high-speed electromagnetic-field sensors.

Yamahata, G., Ryu, S., Johnson, N. et al. Picosecond coherent electron motion in a silicon single-electron source. Nat. Nanotechnol. 14, 1019–1023 (2019) doi: 10.1038/s41565-019-0563-2